carboxylic acids and esters

general formula and naming esters

• general formula: R-COO-R’

• RHS: prefix

• LHS: suffix

acidity of carboxylic acids

weak acids - only partially dissosciates

solubility of carboxylic acids

soluble in water because they can hydrogen bond to water molecules

Salt formation reactions of carboxylic acids

1. Acid + metal —> salt + hydrogen

2. Acid + alkali —> salt + water

3. Acid + carbonate —> salt + water + CO2

Oxidation of methanoic acid

forming esters reagents, conditions and equation

• esterification/condensation reaction

• carboxylic acid + alcohol ——> ester + water

• H2SO4 catalyst

• heat under reflux

hydrolysis of ester with acids

• ester + water ——> carboxylic acid + alcohol

• reagents: dilute HCl

• condition: heat under reflux

hydrolysis of esters with alkali - sapponification

• ester + alkali ——> carboxylate ion + alcohol

• reagent: excess, aqueous, dilute NaOH

• condition: heat under reflux

Fats and soaps

Fats and soaps are esters of glycerol and long chain carboxylic acids (fatty acids)

• vegetable oils and animal fats can be hydrolysed to give soap, glycerol and long chain carboxylic acids

Glycerol

• forms hydrogen bonds very easily and is readily soluble in water

  • used in cosmetics, foods and glues

Soap

• long chain carboxylic (fatty) acids produced by the hydrolysis of fats

• the polar CO₂⁻ end is hydrophilic and mixes with water

• the long non-polar hydrocarbon chain is hydrophobic and mixes with grease

Biodiesel

biodiesel is a mixture of methyl esters of long chain carboxylic acids

• vegetable oils can be converted to biodiesel by a reaction with methanol in the presence of a strong alkali catalyst

it can be argued that the biodiesel produced from the method is classed as carbon-neutral

• bc any CO2 given off when the biofuel is burnt would’ve been extracted from the air by photosynthesis when the plant grew

however this argument doesn’t take into account

• the energy needed to irrigate the plants, extract the oil or heat the reaction with the methanol mixture

  • if the energy for any of these processes comes from fossil fuels than the biodiesel produced cannot be considered carbon neutral

• also doesn’t take into account the effect on land available for food production

acyl chloride

suffix: -oyl chloride

much more reactive than carboxylic acids

explain why ethanoyl chloride reacts readily with nucleophiles

large charge on carbonyl carbon atom since its bonded to both O and Cl

nucleophiles have electron pairs which can be donated

acid anhydride

suffix - anoic anhydride

similar reactivity to acyl chlorides

reasons for using acid anhydride over acyl chloride

• acyl chloride has violent reaction

• toxic HCl gas released when acyl chloride used

• less corrosive

Reaction of acyl chloride and water

• acyl chloride + water ——> carboxylic acid

• nucleophilic addition-elimination

• reagent: water

• conditions: room temperature

• observations: HCl (g) - white steamy fumes

Reaction of acid anhydride and water

• acid anhydride + water —> carboxylic acid

• nucleophilic addition-elimination

• reagent: water

• conditions: room temp.

• observations: nvc

Reaction of acyl chloride and alcohol

• acyl chloride + alcohol ——> ester

• nucleophilic addition-elimination

• reagent: alcohol

• conditions: room temperature

• observations: HCl (g) - white steamy fumes

Reaction of acid anhydride and alcohol

• acid anhydride + alcohol ——> ester

• nucleophilic addition-elimination

• reagent: alcohol

• conditions: room temperature

• observations: nvc

Reaction of acyl chloride and ammonia

• Acyl chloride + ammonia —> primary amide

• Nucleophilic addition-elimination

• Reagent: ammonia

• Conditions: room temp

• Observations: white smoke of NH4Cl is given off

Reaction of acid anhydride and ammonia

• Acid anhydride + ammonia —> primary amide

• Nucleophilic addition-elimination

• Reagent: ammonia

• Conditions: room temp.

• Observations: nvc

Reaction of acyl chloride and primary amines

• Acyl chloride + primary amine —> secondary amide

• Nucleophilic addition-elimination

• Reagent: primary amine

• Conditions: room temp.

• Observation: nvc

Reaction of acid anhydride and primary amines

• Acid anhydride + primary amine —> secondary amide

• Nucleophilic addition-elimination

• Reagent: primary amine

• Conditions: room temp.

• Observations: nvc

recrystallisation method with reasoning

1. Dissolve the impure compound in a minimum volume of hot solvent

   • The minimum volume is used to ensure the hot solution would be saturated (to obtain saturated solution) and to enable crystallisation upon cooling

2. Filter solution through filter paper quickly

   • To remove any insoluble impurities that will prevent crystals reforming

3. Allow solution to cool and crystals to form

   • cool solution increases yield of crystals

   • yield would be lower if solution was warm

4. Filter off the pure product under reduced pressure

   • The water pump connected to the Buchner flask reduces the pressure and speeds up the filtration

5. Wash the crystals with distilled water

   • To remove soluble impurities

6. Dry the crystals between absorbent paper

Reasons for loss of yield in recrystallisation process

• sample was still wet

• sample lost during recrystallisation

• product left in beaker or glassware

Buchner apparatus

How to determine purity using the melting point

measure the melting point using an oil bath:

• heat the melting point tube in an oil bath, heating slowly near the melting point

  • if pure compound it will have a sharp melting point

State the most likely impurity to cause a compounds melting point to be slightly lower than the data-book value.

Give an improvement to the method so that a more accurate melting point can be obtained.

• water

• press the sample of crystals b/w filter paper